Alex Salmond’s former mentor today promised a “day of reckoning” for big business if Scots vote for independence including the nationalisation of BP, the break-up of the banks and a boycott of John Lewis.

Jim Sillars, who shared a campaign platform with the First Minister two days ago, accused large companies who have spoken out against separation this week of attempting to “subvert” the referendum through “lies and distortions”.

He said they were “in cahoots with a rich Tory Prime Minister” and warned nationalists would have their revenge if there is a Yes vote next Thursday.

The tirade by Mr Sillars, a former SNP deputy leader, came as a new YouGov opinion poll showed the Unionist campaign has regained the lead, by a margin of 52 per cent to 48 per cent, after a three-point swing to the No campaign since the weekend.

Senior nationalists are accusing Downing Street of a “dirty tricks campaign” by supposedly orchestrating the chorus of major employers who have spoken out against separation in recent days.

A letter is expected to be published this weekend in which more major retailers, including Marc Bolland, the chief executive of Marks and Spencer, will express their concern about independence and the impact it would have on prices.

Mr Sillars fell out with the First Minister for many years, becoming his most vocal nationalist critic, but he remained an SNP member and the two have reconciled during the campaign, speaking together at a major press event in Edinburgh on Wednesday.

Speaking in the Scottish capital today, he said: “This referendum is about power, and when we get a Yes majority, we will use that power for a day of reckoning with BP and the banks.

Alex Salmond and SNP veteran Jim Sillars together at Team Yes campaign (Demotix)

“The heads of these companies are rich men, in cahoots with a rich English Tory Prime Minister, to keep Scotland’s poor, poorer through lies and distortions. The power they have now to subvert our democracy will come to an end with a Yes.”

He said BP will have to “learn the meaning of nationalisation” and if it wants access to “monster fields” off Shetland “it will have to learn to bend the knee to a greater power – us, the sovereign people of Scotland.”

“We will be the masters of the oilfields, not BP or any other of the majors,” he said, warning Bob Dudley, its chief executive, that this was not “mere rhetoric”.

Mr Sillars warned Scotland’s bankers their “casino days” were over and their businesses would be split between their retail and investment sections, adding: “If your greed takes the latter down, there will be no rescue.”

New employment laws will force Standard Life to give two years’ warning on redundancies, he said, meaning it could not follow through its plans to move savings, pensions and investment business to England.

He suggested Sir Charlie Mayfield, chairman of the John Lewis Group, had taken “instructions from Cameron” when he warned this week that separation would lead to higher prices in Scotland.

“There is now talk of boycott, and if it happens it will be a management own goal. What kind of people do these companies think we are? They will find out,” he concluded.

Mr Salmond yesterday accused Standard Life, BP and Shell of colluding with David Cameron by issuing warnings about the impact of independence during the Prime Minister’s visit to Scotland on Wednesday. The two oil giants backed expert warnings oil will all but have run out by 2050.

Five Scottish-based banks – Royal Bank of Scotland, Lloyds Banking Group, Tesco Bank, TSB and Clydesdale Bank – announced they would move their base to England if there is a Yes vote.

Andy Clarke, chief executive of Asda, and Lord Wolfson, chief executive of Next, have echoed Sir Charlie’s warning separation would lead to higher prices north of the Border.

Adrian Grace, chief executive of insurance and pensions giant Aegon, told Radio Four’s Today programme it may be “forced” to move staff from its Edinburgh base if the separatists win the referendum.

Sir Ian Cheshire, the chief executive of Kingfisher, which includes the B&Q and Screwfix DIY chains, is behind the letter that is expected to be published at the weekend.

Along with Mr Bolland, the other signatories are expected to include Mr Clarke and John Timpson, boss of the Timpson shoe repair business.

Sky News also reported that major telecoms companies were in talks about the implications of a Yes vote.